Tube heater

ABSTRACT

A coil for use in the pyrolysis of hydrocarbons and the like which has legs of unequal length to compensate for expansion during heating.

United States Patent Arthur W. Francis Glenside, Pa.

Feb. 14, 1969 Jan. 5, 1971 Selas Corporation of America Dresher, Pa.

a corporation of Pennsylvania Inventor Appl. No. Filed Patented Assignee TUBE HEATER 5 Claims, 2 Drawing Figs.

US. Cl 122/510 Int. Cl F22b 37/24 Field of Search 122/356,

References Cited UNlTED STATES PATENTS 10/1958 Leech et a]. 122/356 11/1958 Rehm 122/476 11/1960 l-lochmuth 122/476X 10/1961 Throckmorton et al 122/356 5/1968 Fleischer 122/510 Primary Examiner-Kenneth W. Sprague Attorney-E. Wellford Mason ABSTRACT: A coil for use in the pyrolysis of hydrocarbons and the like which has legs of unequal length to compensate for expansion during heating.

PATENTEU JAN 5197! FIG.

FIG.2

INVENTOR.

ARTHUR W. FRANC!S TUBE HEATER SUMMARY OF THE INVENTION The present invention relates .to tube heaters of the type used in the pyrolysis of hydrocarbons, and more particularly to the configuration of a tube used in such heaters.

In the pyrolysis of feed stocks to produce ethylene and other cracked products, the use of vertical serpentine tubes is increasing. Their use is dictated by high severity conditions now being employed wherein the high temperatures used preclude the use of tube supports within the radiant heating section of such a heater. With vertical tubes the tubes can be supported at the exterior of the heater or supports can be located in a shadowed zone where the radiant heat is of less intensity.

In ethylene pyrolysis, for example, the legs or passes of the vertical serpentine tube are adjusted in temperature so that the temperature is progressively increased from the inlet end to the exit. Thus, the legs are at different temperatures and will expand different amounts. As a result of the different expansion of the legs, severe stresses are set up in the tube which can cause breakage of highly refractory materials such as cast alloy tubes.

It is an object of the invention to provide a serpentine tube that is so constructed and mounted that stresses are reduced to a minimum as the temperature of the tube is changed.

It is a further object of the invention to provide a vertically disposed serpentine coil of tubing in a tube heater that is subjected to a minimum of strain as its temperature is changed during operation of the heater.

In practicing the invention, the tube is fabricated in the form of a serpentine coil so configured that the coolest leg of the coil is longer than the hottest leg. The difference in length is such that the legs will expand to the same length thereby preventing stresses from being set up in the legs themselves as well as at their points of support.

The various features of novelty which characterize my invention are pointed out with particularity in the claims annexed to and forming a part of this specification. For a better understanding of the invention, however, its advantages and specific objects attained with its use, reference should be had to the accompanying drawings and descriptive matter in which I have illustrated and described a preferred embodiment of the invention.

IN THE DRAWINGS:

FIG. 1 shows one form a coil of the invention can take; and FIG. 2 shows another form of the invention.

DETAILED DESCRIPTION Referring to the drawings, numeral 1 indicates the top wall of the radiant section of a tube heater that may be of the type shown in U.S. Pat. No. 3,062,197 In the heater with which this application is concerned; however, the tubes 2 are in the form of a serpentine coil having substantially parallel legs 3, 4, 5, and 6 with the two inner legs being joined at their upper ends by a return bend 7. The outer legs are joined to the adjacent inner legs by means of return bends 8 and 9. The coil is all in one plane. I

In a heater of the type referred to herein, the coil is suspended vertically in the radiant section of the heater from the wall 1 and may be attached to the wall at the upper ends of legs 3 and 6 in any conventional manner, which is shown herein as comprising collars ll encircling these ends and attached thereto. In order to give some support to the inner legs and upper return bend 7, there is provided a yoke 12 fastened to wall I which receives loosely a ring 13 fastened to the return bend. This connection will permit some movement of the bend as the coil changes in dimension.

Fluid to be heated is supplied to the coil through leg 3 and is discharged from the upper end of leg 6. The latter leg is heated to a higher temperature than leg 3 because the fluid gradually increases in temperature as it moves through the coil. The-hottest leg 6 has a shorter effective length in the radiant section of the furnace than does the leg 2. In FIG. 1 the insulation of wall 1 is shown at 14 as being substantially thicker where leg 6 passes through it than it is where leg 3 passes through it. In FIG. 2 a shorter effective length of leg 6 is attained by forming a depressed portion 15 in top wall I where the upper end of leg 6 extends through this wall.

In the operation of a tube heater the temperature to which the fluid being treated must be raised, is known. This, in turn, dictates the temperature of the radiant section of the heater and of the coil. In constructing the coil, leg 3 must have an effective length, or length subjected to the direct heat of the radiant section of the heater, relative to the length of leg 6 such that leg 3 times its ratio of linear expansion equals leg 6 times its ratio of linear expansion at temperature. Since leg 6 is hotter than leg 3, it will expand more and therefore, to keep theexpansion of both legs the same, must be shorter than leg 3. i

in FIG. 1 the shorter effective length of leg 6 is obtained by increasing the thickness of the insulation in wall 1 so that only the desired length of the leg will be exposed to heat. In FIG. 2 the same result is obtained by forming a depression in the roof where the" upper end of leg 6 extends through it. In either case the exposed or effective length of leg 6 will expand substan- 'tially. the same amount as the longer length of leg 3. Thus, the

stresses induced in the coil are substantially reduced. In addition, there will be no tendency for the coil to bend where it enters the heater thereby reducing strain at its fixed points.

Over a period of time the heat-induced stresses in the tubular coil will relieve themselves progressively by creep of the metal. The exact hot configuration will therefor differ from the cold configuration. When the coil is cooled a reverse stress will occur. This can be avoided if the coil is so shaped that the cold and hot configurations are substantially the same. To this end the bends 8 and 9 between the lower ends of the vertical, parallel legs of the coil are formed in the shape of a catenary. A curve of this shape placed in the coil when it is formed results in a minimum of creep and stress as the coil is heated up and cooled down.

In summary, for purposes of minimum stress and maximum life ofa coil, the vertical legs should be of unequal length with the coldest leg the longest, and the bends between the bottoms of the adjacent legs should be in the shape of a catenary. While the coil has been shown as having four legs, it will be obvious that it can have any number of legs desired as long as the principals of the invention are followed.

While in accordance with the provisions of the Statutes I have illustrated and described the best form of embodiment of my invention now known to me, it will be apparent to those skilled in the art that changes may be made in the form of the apparatus disclosed without departing from the spirit and scope of the invention set forth in the appended claims, and that in some cases certain features of my invention may be used to advantage without a corresponding use of other features.

I claim:

1. In a tube heater, structure forming a substantially horizontal upper wall of a radiant heat chamber, a tube in the shape of a serpentine coil having substantially parallel and a spaced apart legs joint joined by bends, the end portions of said coil being straight and extending through said wall, means cooperating with said straight end portions to suspend said coil from said wall with the coil in a vertical plane, fluid flowing through said coil being heated as it moves through said coil from one leg to another, the leg of said coil through which heated fluid leaves the coil having an effective length shorter than the leg the fluid enters.

2. The combination of claim 1 in which adjacent lower ends of said legs of said coil are joined by curved sections in the shape of a catenary.

3. The combination of claim 1 in which said wall has a low portion at the location where the leg having the shorter effective length passes through it.

. I ail 4. The combination-f claim 1 in which said wallils gulps mnlgggga'idzl 'wiqg ingvgpiclil plhfi;aliepum bcnd connectjngathelqgple 'n'dabmhe innerlgs, retufn 'bhdvinthe fonn jgiallythi'ckeria t the location when: thellcg haiving tfll ,ihpfter r *dther leg passes through said wall. l 5 

1. In a tube heater, structure forming a substantially horizontal upper wall of a radiant heat chamber, a tube in the shape of a serpentine coil haviNg substantially parallel and a spaced apart legs joint joined by bends, the end portions of said coil being straight and extending through said wall, means cooperating with said straight end portions to suspend said coil from said wall with the coil in a vertical plane, fluid flowing through said coil being heated as it moves through said coil from one leg to another, the leg of said coil through which heated fluid leaves the coil having an effective length shorter than the leg the fluid enters.
 2. The combination of claim 1 in which adjacent lower ends of said legs of said coil are joined by curved sections in the shape of a catenary.
 3. The combination of claim 1 in which said wall has a low portion at the location where the leg having the shorter effective length passes through it.
 4. The combination of claim 1 in which said wall is substantially thicker at the location where the leg having the shorter effective length passes through it than the location where the other leg passes through said wall.
 5. The combination of claim 1 in which the coil has four legs, said legs being in a vertical plane, a return bend connecting the upper ends of the inner legs, return bends in the form of a catenary connecting the lower ends of the outer legs to the lower ends of the adjacent inner legs. 